The invention relates generally to continuous manufacture of steel tubing, and relates more particularly to finishing operations.
Due to its susceptibility to corrosion, steel tubing is often coated with one or more layers of protective materials. For example, steel tubing may be galvanized by application of molten zinc, or may be coated with various other paints or coating materials, alone or in combination. It has long been known in the art that significant economies may be realized by applying coatings to the tubing in line with manufacture thereof, rather than applying coating materials in separate operations after the tubing has been cut to length. (See, e.g., application Ser. No. 08/026,432 assigned to the same assignee as the present application filed Mar. 4, 1993.)
A problem which arises in connection with in-line coating of steel tubing is that coating thickness is often difficult to control at production speeds. Air knives have been employed to blow excess coating material from the tubing, but air knives do not permit a great deal of precision to be maintained in regulating coating thickness.
Typically, coating processes require that coating thickness not fall below a minimum value needed for adequate protection of the substrate. Maximum permissible thickness may be a function of cost. That is, excessive thickness represents waste of coating material. Also, excessive thickness may make the product unacceptable by leaving visible irregularities on the product surface due to running of coating material, or by leading to insufficient drying or curing of the coating within the time available.
It is a general object of the invention to provide an improved method and apparatus for in-line coating of steel tubing to provide improved control of coating thickness.